Field of Invention
This invention relates to machinery for processing solid material and, in particular, to a monitor for monitoring acoustic emission signals in the machinery to detect rubbing of rotating or vibrating parts contained in the machinery.
Machinery is generally known for processing solid material that is in the form of a powder, crystals, granules, chips, flakes, pellets and the like. For example, machinery used to process aspirin includes a granulator, a sifter and a large and a small screw conveyor. This machinery involves the relative motion of two metallic parts with a narrow clearance in between. One part is usually stationary (for example, a screen, a trough or a casing) and the other has a rotating or vibrating motion (such as a rotor or a screw). Rubbing occurs between the rotating and stationary parts when there are changes in the parts due to use and/or wear.
The granulator breaks compacted aspirin chunks into small granules using oscillating steel blades to force the chunks through a wire screen. The blades are located close enough to the screen to make rubbing or wear contact that produces metallic particles that contaminate the aspirin material.
The sifter is a generally cylindrical container standing on springs and connected to a motor. The container is divided into vertical segments by large screens that are progressively finer mesh from top to bottom, so as aspirin granules enter the sifter from the top and fall through the screens, the granules are sorted by size. When operating the container is shaken by the motor and bounces on the springs in order to agitate the granules. Screen wire may break during agitation and make rubbing contact that produces metallic particles that contaminate the aspirin material.
A screw conveyor is used to provide horizontal transport of aspirin material. The material falls into a U-shaped tube from above and is pushed along the tube by a rotating screw having bearings at either end.
The bearings can wear and result in rubbing contact at the bearings as well as between the screw and the U-shaped tube. The latter rubbing contact produces metallic particles that contaminate the aspirin material.
Magnetic traps have been used to capture the metallic particles. However, these traps are unable to assure a 100% metal free product due to very fine and/or poorly magnetic particles and are further unable to determine what machine or part of a machine is causing the contamination.
Devices are known that detect sound or acoustic emission signals developed in machinery having rotating or vibrating parts. Acoustic emission signals are elastic waves or stress waves emitted in or at the surface of a material with frequencies in the ultrasonic domain, typically 20 kilo Hertz (KHz) to 2 mega Hertz(MHz). Acoustic emission signals are short transients (durations from about 100 microseconds (xcexcs) to 100 milliseconds (ms)) usually emitted in large numbers (few hundreds to few millions). For instance, most acoustic emission instruments are able to acquire over 1,000 signals per second that correspond to sudden releases of energy from rubbing or any other cause.
A device that uses acoustic emission signals to detect rubbing locations in machinery having rotating parts is described in U. S. Pat. No. 4,377,947. This device employs two spaced apart sensors, a pulse generator that generates a pulse for each rotation of the rotating part and a circuit for identifying a rubbing location on the basis of the travel time difference of acoustic emission signals arriving at the two sensors. This device does not have any filter to remove noise from the sensed acoustic emission signals.
A device that uses acoustic emission signals to detect cracks in machinery having a rotating body is described in U.S. Pat. No. 4,685,335. The device employs two pairs of sensors arranged on the machinery for detecting acoustic signals. The device includes means to remove noise of certain types from the sensed acoustic emission signals. Thus, noise due to rubbing and due to metal wiping is recognized and removed from the sensed acoustic emission signals. The device is not used for processing solid material and, therefore, does not remove noise that affects recognition of rubbing in a solid material processing environment.
Accordingly, there is a need to recognize and locate rubbing in solid material processing machines.
A main object of the present invention is to provide an apparatus that identifies rubbing signals in acoustic emission signals produced in a solid material processing machine.
Another object of the present invention is to provide an apparatus that identifies rubbing signals in a large number of acoustic emission signals produced in a solid material processing machine.
Yet another object of the present invention is to provide a method that identifies rubbing signals in a large number of acoustic emission signals produced in a solid material processing machine.
A further object of the present invention is to provide an apparatus that filters from acoustic emission signals operating noise that affects recognition of rubbing signals.
A still further object of the present invention is to provide a memory media for controlling a computer system to identify rubbing signals in acoustic emission signals produced in a solid material processing machine.
In accordance with the present invention, apparatus is provided to identify rubbing signals in acoustic emission signals that are produced in a machine that processes solid material. The apparatus includes a filter for filtering operating noise of the machine from the acoustic emission signals to produce filtered signals. The operating noise filtered out includes signals that have an acoustic emission activity (or recurrence rate) greater than the operating rate of the rotating or vibrating part of the machine, an acoustic emission amplitude less than a front end filter threshold amplitude and a signal duration less than a front end filter threshold signal duration. A discriminator detects rubbing signals from the filtered signals according to the following characteristics: acoustic emission intensity greater than a predetermined intensity, acoustic emission activity greater than the machinery operating rate and acoustic emission signal length greater than a predetermined length. Signaling means responds to the detected rubbing signals to signal occurrence of rubbing.
In a specific embodiment of the present invention, the front end filter threshold amplitude is in the range of about 40 to 55 decibels AE and the front end filter threshold signal duration is about 1 to 10 ms.
According to one embodiment of the present invention, the filter, discriminator and signaling means are configured in a computer by an acoustic emission signal program and a rubbing signal program. The rubbing signal program configures the computer as a means to establish the operating noise filter conditions for the filter. The filter conditions are first set for the machine running unloaded and then adjusted for the machine running solid material in a well maintained and non-rubbing condition.
In alternate embodiments, the discriminator detects the rubbing signal intensity from at least one of the group that includes: peak amplitude, RMS voltage, relative energy and true energy. The signal length is detected from at least one of the group that includes: duration, rise time, counts to peak, event counts and average frequency.
A memory media according to the present invention controls a computer to identify rubbing signals, wherein the computer is configured to perform filter operations on acoustic emission signals. The memory media includes:
(a) means for configuring said computer system to set filter conditions for filtering operating noise from the acoustic emission signals to form the filtered signals, the filtered out operating noise including signals that have an acoustic emission activity greater than said operating rate, an amplitude less than a front end filter threshold amplitude and a signal duration less than a front end filter threshold duration;
(b) means for configuring the computer system to detect the rubbing signals from the filtered signals, the rubbing signals including the following characteristics: acoustic emission intensity greater than a predetermined intensity, acoustic emission activity greater than the operating rate and acoustic emission signal length greater than a predetermined length; and
(c) means for configuring the computer system to signal detection of the rubbing signals.
According to a more specific embodiment the memory media of the present invention, means (a) is operable to first set the filter conditions for the machine running unloaded and then to adjust the set filter conditions for the machine running solid material in a well maintained and non-rubbing condition.
The method according to the invention includes:
producing electrical signals proportional to acoustic signals;
filtering operating noise from the electrical signals to provide filtered signals, the operating noise filtered out including signals that have an acoustic emission activity greater than the operating rate, an amplitude less than a front end filter threshold amplitude and a signal duration less than a front end filter threshold signal duration;
detecting the rubbing signals from the filtered signals, the rubbing signals having the following characteristics: acoustic emission intensity greater than a predetermined intensity, acoustic emission activity greater than the operating rate and acoustic emission signal length greater than a predetermined length; and
signaling the occurrence of detected rubbing signals.
In a more specific embodiment, the method according to the present invention further includes setting filter conditions for the filtering step by first setting the filter conditions for the machine running unloaded and then adjusting the set filter conditions for the machine running solid material in a well maintained and non-rubbing condition.
In a specific embodiment of the method according to the present invention, the front end filter threshold amplitude is in the range of about 40 to 55 decibels AE and the front end filter threshold signal duration is about 1 to 10 ms.
In alternate embodiments of the method according to the present invention, the rubbing signal intensity is detected from at least one of the group that includes: peak amplitude, RMS voltage, relative energy and true energy. The acoustic emission signal length is detected from at least one of the group that includes: duration, rise time, counts to peak, event counts and average frequency.